Welding is the last operation during fabrication of many high value added products. Manufacturers of such products need welding processes/technologies to join metals with acceptable quality at as low as possible costs in order to be competitive. To this end, gas metal arc welding (GMAW) and its variants such as submerged arc welding (SAW) and flux cored arc welding (FCAW) have become the most widely used processes for mechanized and automated joining of metals especially in heavy industry where the cost of welding is critical. They are selected over other possible processes for joining metals such as gas tungsten arc welding (GTAW), laser and electron beam welding, friction welding, resistive welding, etc. because they are cost effective for vast majority of applications in terms of fast melting of filler metal, easy operation and maintenance, relatively low requirement on joint preparation and working conditions, applicability for many joint designs, relatively low requirement on labor skills, tolerance on process variations, robustness and consistence in weld quality, relatively low equipment and capital investment, etc.
Despite the overall advantage of GMAW and its variants over other processes, there are areas where modifications are needed to further reduce costs. For many applications, the welding time may need to be as small as possible in order to reduce the overall production cycle. First, modifications are needed for GMAW and its variants to further improve the welding productivity by further increasing the welding speed and reducing the number of passes. To this end, the wire needs to be melted at as high as possible speed. Second, for conventional GMAW and its variants which melt wires by the arc established between the wire and work-piece, the arc also heats the work-piece in addition to the wire. The arc heat which directly heats the work-piece, referred to as excessive heat hereinafter, is proportional to the arc heat which melts the wire, referred to as effective heat hereinafter. While the effective heat is always desirable for melting the wire, the excessive heat needs to be controlled at desirable level to achieve desirable level of fusion and control the material properties of the resultant welds; the excessive heat input accumulated over all passes, referred to as accumulative excessive heat input hereinafter, is desired to be as low as possible to control the distortion which is generally considered to increase as the accumulative heat input. Hence, modifications are also needed to control the excessive heat input at desired level and reduce the accumulative excessive heat input as much as permitted.
What are desired are methods to modify the GMAW and its variants such that the modified process can reduce the accumulative heat input and distortion; control the heat input at the level in a particular pass to produce acceptable materials properties; increase the melting speed with only minimal excessive heat input increase; and be incorporated easily with existing GMAW equipment/investment because of the massive use of GMAW and its variants.
Before a solution can be discussed, some problems in GMAW and its variants need to be analyzed. In conventional GMAW shown in FIG. 1, an arc 11 is established between the wire 12 and base metal 13 and the current 14 which heats the wire is exactly the same as the current 15 which heats the base metal (like in all conventional arc welding processes). This fundamental characteristic of conventional arc welding produces unwanted side-effects to all arc welding processes including the GMAW and its variants.
First, in GMAW and its variants, the wire 12 is primarily melted by the anode heat IVanode but the cathode heat IVcathode is imposed totally on the base metal 13 (workpiece). The effective and excessive heat in conventional GMAW are thus IVanode and IVcathode respectively. Because Vcathode is greater than Vanode in GMAW and its variants, IVcathode>IVanode. Moreover, the excessive heat IVcathode proportionally increases with the effective heat IVanode and increasing the current to increase the effective heat for melting the wire 12 faster will proportionally increase the excessive heat contributing to enlarging the weld pool 16, degrading the materials properties of the weld and the heat-affected zone (HAZ).
Second, the arc pressure is given as
      P    arc    =                    μ        0            ⁢      IJ              4      ⁢                          ⁢      π      where μ0 is permeability in vacuum space, I is the welding current, J is the current density. It is understood that the current density follows a Gaussian distribution
      J    ⁡          (      r      )        =            I              2        ⁢                                  ⁢        π        ⁢                                  ⁢                  σ          j          2                      ⁢          exp      ⁡              (                  -                                    r              2                                      2              ⁢                                                          ⁢                              σ                j                2                                                    )            where σj is current distribution parameter and r=√{square root over (x2+y2)}. As a result, the arc pressure can be expressed as
      P    arc    =                              μ          0                ⁢                  I          2                            8        ⁢                                  ⁢                  π          2                ⁢                  σ          j          2                      ⁢                  exp        ⁡                  (                      -                                          r                2                                            2                ⁢                                                                  ⁢                                  σ                  j                  2                                                              )                    .      The maximum pressure is
      P          arc      ,      max        =                              μ          0                ⁢                  I          2                            8        ⁢                                  ⁢                  π          2                ⁢                  σ          j          2                      .  As the cathode of the arc, the weld pool 16 on the base metal 13 is always subject to an arc pressure proportional to the square of the current. This suggests that in addition to increasing the weld pool 16, increasing the current 14 also increases the arc pressure rapidly. Using a large current to increase the melting speed or deposition rate therefore also increases the risk to burn-through the base metal 13, generate undercuts, and blow liquid metal away from the weld pool 16.
Conventional GMAW and its variants do not have the capabilities to reduce the accumulative heat input and distortion because (1) the needed accumulative effective heat is determined by the weld size or the metal which needs to be melted which are both fixed for a given application; (2) the accumulative excessive heat is proportional to the accumulative effective heat in GMAW and its variants and is also fixed; (3) the sum of the accumulative effective heat and excessive heat is thus also fixed and the accumulative heat input is thus fixed. It is possible to control the heat input at the level in a particular pass to produce acceptable materials properties but the side effect is that the deposition rate will be fixed because (1) the heat input consists of the effective and excessive heat; (2) when the heat input is fixed, the effective heat is also fixed because the excessive heat and effective maintains a fixed proportion; (3) when the effective heat is fixed, the melting speed of the wire 12 is also fixed. As a result, controlling the heat input at a desired level (typically relatively low) would result in a low deposition rate each pass and the needed number of passes would be large resulting in a low productivity. It is not possible to increase the melting speed without or only with minimal excessive heat input increase because (1) the melting speed is approximately proportional to the effective heat and (2) the excessive heat input is proportional to the effective heat. In short, increasing the melting speed will proportionally increase the excessive heat Finally, increasing melting speed will increase the current 14 and the arc pressure will increase rapidly causing additional problems.
It can be easily appreciated that the fundamental characteristic of the GMAW and its variants, (i.e., the melting current 14 which determines the effective heat and melting speed equals the base metal current 15 which determines the excessive heat and arc pressure) is a significant reason for the root of the problems listed above. Modifications need to be taken to differentiate those two currents but the modifications should be easily incorporated with existing GMAW and its variants' equipment/investment because of the massive use of GMAW and its variants in addition to being cost effective.
It should be pointed out that several other technologies have been developed to modify GMAW process from different aspects. Among these technologies, two have received attention: tandem GMAW and Variable-Polarity GMAW (VP-GMAW). In Tandem GMAW, two torches have been integrated into one bigger torch, and two close parallel arcs are adjusted by two GMAW power supplies independently. In essence, Tandem GMAW is still considered two parallel GMAW processes, but it can alternate the maximum welding current to each torch. As a result, the arc pressure remains unchanged, and the wire feed speed can be doubled. Hence, if arc pressure is the major concern, Tandem GMAW can double the deposition rate. However, this modification does not reduce the heat input. The ratio between the excessive heat and effective heat remains unchanged from the conventional GMAW. For VP-GMAW, liquid droplets are still detached during the reverse polarity (wire positive) period, but the welding wire can be melted faster during the straight polarity (wire negative) period. It was found that to melt the welding wire at the same rate, the base metal heat input could be “up to 47 percent less” than the conventional pulsed GMAW. The principle is that in conventional GMAW, it is always true that the effective heat is VanodeI and the excessive heat is VcathodeI; however, in VP-GMAW, VanodeI and VcathodeI alternate its role as the effective and excessive heat. Because VcathodeI>VanodeI, this alternation reduces the excessive heat input and increases the effective heat. However, because the current is the same for the generation of the effective heat and excessive heat, it does not appear to be a process which can freely control them to reduce the excessive heat as much as desired.